The present invention relates to a hydraulic circuit for a lock-up device.
In Japanese Unexamined Patent Publication (Kokai) No. 61-105353, there is disclosed a conventional hydraulic circuit for the lock-up device, which includes a fluid coupling with a lock-up mechanism and which is used for an infinite variable-speed drive.
In the conventional circuit, the interior of the fluid coupling is divided into an "apply" chamber and a release chamber. In an inactivated state of the lock-up device, constant pressure adjusted by a coupling-pressure regulating valve is introduced into the apply chambers and the release chamber through a lock-up valve, so that the hydraulic pressures of the apply chamber and release chamber are maintained and are equal to each other.
In an activated state of the lock-up device, by changing the lock-up valve arranged in the hydraulic circuit and thereby connecting an oil path for supplying the hydraulic pressure into the release chamber with a drain, a rise of the pressure in the apply chamber in excess of the pressure in the release chamber allows a lock-up clutch to be joined for the lock-up state. Hereat, it is noted that the joint force of the lock-up clutch depends on a differential pressure between the pressure in the apply chamber and that in the release chamber.
In the middle of the oil path for supplying the hydraulic pressure to the apply chamber, another oil path is arranged to flow the hydraulic oil into the oil cooler. Provided in the middle of the oil path for flowing the oil into the oil cooler is a regulating valve which flows out the hydraulic oil for the oil cooler, when the hydraulic pressure in the apply chamber is more than a constant value. This regulating valve also serves to stop the flow out of the hydraulic oil for the oil cooler because there is a possibility that insufficient hydraulic pressure will be supplied for the apply chamber in case that a vehicle-speed is low and the discharging amount of the oil pump is small. The hydraulic oil cooled by the oil cooler is introduced into the lubricating circuit.
As mentioned above, the joint force of the lock-up clutch depends on the differential pressure between the hydraulic pressure in the apply chamber and the hydraulic pressure in the release chamber. Therefore, the hydraulic pressure of the apply chamber corresponding to the engine torque is necessary to execute such a lock-up.
However, because the hydraulic pressure in the apply chamber is maintained below the constant pressure by the regulating valve, it is impossible to supply the hydraulic pressure necessary for the lock-up required in a range of large engine torque. Further, there is caused a problem that, if increasing the constant pressure maintained by the regulating valve, the inflow to the oil cooler will be reduced, so that the hydraulic oil cannot be sufficiently cooldown and the oil for lubrication becomes insufficient.